I was recently looking to educate myself a little more about this topic and found this article on accessibility which summarises some of the key points. So even if you aren’t going for the top AAA standard (apparently fairly hard to attain), this is a good starting point for the basics: http://www.accessible.org/accessible-web-designs.html.

Examples include:

Making sure buttons make sense by themselves i.e. if it says ‘click here’ a partially sighted person tabbing through content won’t know what they are clicking to see. So better to say ‘click here for news’.

Making sure the images and text are enlargeable.

Making sure images and maps have alt tags.

Etc….

The guidelines come from the Web Accessibility Initiative (WAI) Guidelines – which I believe is the main document everyone referts to for AAA standards and has lots of detail in it: http://www.w3.org/WAI/